Canon
Speedlite 580EX Flash Durations:How
long the flash is on for powers settings down to 1/128.

Electronic
flashes vary power output by changing the length of time the flash
is on. Knowing the duration of the flash is very useful for
estimating what duration is needed for stopping the action when the
velocity of the object to be frozen is known. It's also necessary
when the flash duration and blur are used to measure the speed.

Since
these are both things I do from time to time I was understandably
disappointed to discover that the manual for the Canon Speedlite
580EX flash did not include the flash durations for the various power
settings. I sent Canon two emails requesting this information but
while they responded to the emails they said they couldn't find the
information. I then called Canon and was told, "that
information is not available," and the technical
representative refused to dig any deeper for it. (This surprised me
because just a week earlier I called them with a lens question and
when the tech rep. couldn't answer it she walked me through two
levels of engineers until we found someone who could. It appears the
quality of Canon's technical assistance is dependent on the mood of
the person answering the call.)

It
is a well known axiom that,"Hell hath no fury like an
engineer scorned." Frustrated by Canon's inability or
refusal to provide this information I determined to figure it out myself.

The
process turned out to be simple. I looked around the house and found
the fastest moving thing I could lay my hands on: a rotary saw.
Knowing the blade radius and rotational speed I calculated that the
teeth were traveling at 1658 inches per second. By taking pictures of
the teeth at various flash power settings and measuring the length of
blur I was able to approximate the duration of the flash.

The tooth gap captured at 1/128th power.

The
lowest power setting I could go was 1/16 before the blur closed the
tooth gap and made measurements impossible. Fortunately, the data
from tests at 1/16, 1/32, 1/64, and 1/128 power were enough to get a
good idea of the flash durations. Four shots were taken at each power
setting, the blur lengths measured for each shot, and the values
statistically averaged to find the estimated flash duration for each
power setting. The calculated values for these four settings were:

1/16
power = 1/15,000 second

1/32
power = 1/19,000 second

1/64
power = 1/31,000 second

1/128
power = 1/35,000 second

Inaccuracies
in measuring the blur and speed variations of the blade indicates
that these values are accurate to plus-or-minus 25 percent. So, the
flash duration for the 1/128 power setting is somewhere between
1/26,000 and 1/44,000 second. (I admit this isn't very precise but
it's a lot better than I got from Canon.)

As
any engineer I love graphs so here's one I drew up for the Speedlite:

The
black line represents what the flash duration should be if the unit
was constant in it's efficiency. At full power, power setting 1.0,
the manual states that the flash is 1/833 second. If the efficiency
remained constant then the duration at 1/2 power would be 1/1667
second and so on to the 1/128 power setting, which would have a
duration of only 1/107,000 second. However, in the real world as the
flash duration for xenon flash lamps becomes very small their
efficiency decreases. To compensate for this loss of light the flash
is programmed to remain on long enough so that the output is what it
should be.

The
red line in the graph represents my data.

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